The invention relates generally to a method of determining the direction a tool face points; more particularly, to determining the orientation and/or angular displacement of a tiltable joint of a bottom hole assembly.
Steerable systems for use drilling boreholes in a formation, for example, for subsequent use in the extraction of oil or gas, are well known. One steerable system is a rotary steerable drilling system, which can include substantially continuous rotation of the drill string. Rotary steerable systems can be classified as “point-the-bit” systems, “push-the-bit” systems, or even a hybrid system, such as described in U.S. Pat. No. 7,188,685 entitled Hybrid Rotary Steerable System. Examples of point-the-bit type rotary steerable systems and how they operate are described in U.S. Patent Application Publication Nos. 2002/0011359; 2001/0052428 and U.S. Pat. Nos. 6,394,193; 6,364,034; 6,244,361; 6,158,529; 6,092,610; and 5,113,953, all herein incorporated by reference. Examples of push-the-bit type rotary steerable systems and how they operate are described in U.S. Pat. Nos. 5,265,682, 5,553,678; 5,803,185; 6,089,332; 5,695,015; 5,685,379; 5,706,905; 5,553,679; 5,673,763; 5,520,255; 5,603,385; 5,582,259; 5,778,992; 5,971,085 all herein incorporated by reference.
Regardless of the type of steerable system, a bottom hole assembly of a drilling system can include a tiltable joint. This joint can be used, for example, to aim a tool face in a desired direction which can control the direction in which the borehole propagates. The movement of the joint relative to the bottom hole assembly, e.g., the direction in which the tiltable joint points, is primarily controlled by the force applied by steering actuators, which can be drilling fluid powered. These forces can be referenced with respect to a formation fixed frame work, instead of with respect to the rotating bottom hole assembly, and so the direction in which the actuators apply force to point the tiltable joint can be inertially referenced.
Unknown forces, for example, bottom hole dynamics, bending, frictional contact of the bottom hole assembly with the formation, drill bit reaction forces, joint friction, weight on bit, etc., act to perturb the direction in which the tiltable joint, e.g., the tool face, points. It can be desirable to determine the direction a tool face points, or more particularly, to determine the orientation and/or angular displacement of a tiltable joint of a bottom hole assembly.
The orientation and/or angular displacement of the tiltable joint with respect to the bottom hole assembly can be directly measured by a resolver or angular potentiometer on the tiltable joint and/or gap-sensors measuring relative motions in two non-collinear planes (inductive, capacity, etc.) between tiltable joint and bottom hole assembly. However inclusion of such devices can be impossible or undesirable, e.g., tight tolerances.